The present invention relates to a process for the conversion of an olefinic C.sub.4 cut to polyisobutene and to propylene. Part or all of the C.sub.4 cut may originate from the conversion of an olefinic C.sub.5 cut. When the cuts originate from a steam cracking operation, a further aim of the present invention is to optimise the relative ethylene-propylene selectivity of the steam cracking procedure using this process.
Steam cracking of feeds constituted by light paraffin cuts supplies ethylene and propylene for the petrochemical industry. It also provides a number of other heavier products, in particular a C.sub.4 hydrocarbon cut which contains mainly 1,3-butadiene, isobutene, n-butenes and butanes, accompanied by traces of acetylenic hydrocarbons, and a C.sub.5 hydrocarbon cut which contains mainly C.sub.5 diolefins, methylbutenes, n-pentenes and pentanes, accompanied by traces of acetylenic hydrocarbons.
Likewise, catalytic cracking of heavy hydrocarbon feeds, in particular fluid catalytic cracking (FCC), produces lighter products, among them a C.sub.4 hydrocarbon cut which contains mainly isobutane, isobutene, n-butenes and butanes, accompanied by small quantities of 1,3-butadiene and acetylenic hydrocarbons, and a C.sub.5 hydrocarbon cut which contains mainly pentanes, methylbutenes and n-pentenes, accompanied by small quantities of C.sub.5 diolefins and acetylenic hydrocarbons.
Until recently, only the 1,3-butadiene and the isobutene were of use in the polymer industry, in particular in the tire industry for the former. An increase in tire lifetime and a relative stagnation of demand has led to a surplus of butadiene which is not used to any great extent. In contrast, isobutene has gained in importance as it can be used to synthesize polymers with many uses.